Although the Cu doped Bi2Se3 topological insulator was discovered and intensively studied for almost a decade, its electrical and magnetic properties in normal state, and the mechanism of "high-Tc" superconductivity regarding the relatively low carrier density are still not addressed yet. In this work, we report a systematic investigation of magnetic susceptibility, critical fields, and electrical transport on the nominal Cu0.20Bi2Se3 single crystals with Tc^onset=4.18 K, the highest so far. The composition analysis on the sample yields the Cu stoichiometry of x=0.09(1). The magnetic susceptibility shows considerable anisotropy and an obvious kink at around 96 K was observed in the magnetic susceptibility for the magnetic field parallel to c-axis (H∥c), which indicates a charge density anomaly. The electrical transport measurements indicate the two-dimensional (2D) Fermi liquid behavior at low temperatures with a high Kadowaki-Woods ratio, A/γ^2=30.3*10-5 μΩ cm mol^2 K^2/mJ^2=30.3a0. The lower critical field at 0 K limit was extracted to be 6.0 Oe for the magnetic field perpendicular to c-axis (H∥ab). In the clean limit, the ratio of energy gap to Tc was determined to be ∆0⁄(kB Tc )=2.029+-0.124 exceeding the standard BCS value 1.764, suggesting that Cu0.09Bi2Se3 is a strong-coupling superconductor. The in-plane penetration depth at 0 K was calculated to be 1541.57 nm, resulting in an unprecedented high ratio of Tc/λ^(-2) (0)9.86. Moreover, the ratio of Tc to Fermi temperature (TF) is estimated to be Tc⁄(TF^2D )=0.034. Both ratios fall into the region of unconventional superconductivity according to Uemura's regime, supporting the unconventional superconducting mechanism in CuxBi2Se3. Finally, the enhanced Tc value higher than 4 K is proposed to arise from the increased density of states at Fermi energy and strong electron-phonon interaction induced by the charge density instability. Keywords: topological material, critical parameters, unconventional superconductivity.

中文翻译：

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来自磁化率和电传输的 CuxBi2Se3 中的非常规超导性

尽管铜掺杂的 Bi2Se3 拓扑绝缘体被发现并进行了近十年的深入研究，但其在正常状态下的电学和磁学特性以及相对较低载流子密度的“高 Tc”超导机制仍未得到解决。在这项工作中，我们报告了对标称 Cu0.20Bi2Se3 单晶的磁化率、临界场和电传输的系统研究，Tc^onset=4.18 K，是迄今为止最高的。对样品的成分分析得出 x=0.09(1) 的 Cu 化学计量。磁化率表现出相当大的各向异性，在平行于 c 轴（H∥c）的磁场中，在 96 K 附近观察到明显的扭结，表明电荷密度异常。电输运测量表明低温下二维 (2D) 费米液体行为具有高 Kadowaki-Woods 比，A/γ^2=30.3*10-5 μΩ cm mol^2 K^2/mJ^2= 30.3a0。对于垂直于 c 轴 (H∥ab) 的磁场，在 0 K 极限处提取的下临界场为 6.0 Oe。在清洁极限下，能隙与 Tc 的比值确定为 ∆0⁄(kB Tc )=2.029+-0.124，超过标准 BCS 值 1.764，表明 Cu0.09Bi2Se3 是强耦合超导体。计算出 0 K 时的面内穿透深度为 1541.57 nm，导致 Tc/λ^(-2) (0)9.86 的比率达到前所未有的高。此外，Tc 与费米温度 (TF) 的比率估计为 Tc⁄(TF^2D )=0.034。根据 Uemura 的制度，这两个比率都属于非常规超导区域，支持 CuxBi2Se3 中的非常规超导机制。最后，提出高于 4 K 的增强 Tc 值是由于费米能级的态密度增加和电荷密度不稳定性引起的强电子 - 声子相互作用引起的。关键词：拓扑材料，临界参数，非常规超导。